1. Field of the Invention
The present invention relates to a method of producing a deflection yoke core used for color TVs, display monitors and so on.
2. Discussion of Background
FIG. 4 is a front view partly broken of a typical deflection yoke core. In FIG. 4, reference numeral 1 designates a deflection yoke core having a flare shape which is formed by sintering a molded body made of ferrite powder, numeral 2 designates parting grooves formed in the outer circumferential surface and the inner circumferential surface of the deflection yoke core 1 at positions opposing to each other so that the deflection yoke core 1 can be divided into two portions, numerals 3, 4 designate core halves splitted at the parting grooves 2 and numerals 5 and 6 designate openings for clipping which are formed in the core halves 3, 4 at both sides of the parting grooves 2.
The deflection yoke core 1 is divided into the core halves 3, 4 at the parting grooves 2 after the calcination of the core 1, each of the core halves 3, 4 is wound with a wire. Then, the core halves are combined into a single body. Clips are used to clip the opposing grooves 5, 6 for clipping. Thus, the deflection yoke core 1 is assembled.
In a conventional method of producing the above-mentioned molded product for the deflection yoke core 1, a metal mold in which projections are previously formed at positions corresponding to the parting grooves 2 and openings 5, 6 for clipping, is used. Ferrite powder as a raw material is put into the metal mold, and the powder is pressurized to thereby form a molded product having the parting grooves 2 and the openings 5, 6 for clipping. The molded product is calcined at a high temperature of a thousand and several hundreds centigrades to thereby obtain the deflection yoke core 1.
The deflection yoke core 1 obtained by the above-mentioned pressing method has a disadvantage that it is easily deformed in the calcination due to its having a small specific gravity and uneven packing density to thereby cause poor dimensional accuracy. In order to improve such a disadvantage, an attempt of employing a wet molding method with use of a lathe has been made.
FIG. 5 is a diagram showing a method of shaping with the use of a lathe. In FIG. 5, reference numeral 21 designates a bowl-shaped plaster mold, numeral 22 designates a clay as raw material for a molded product, numeral 23 designates a rotating trowel made of metal, numeral 24 designates a deburring knife and numeral 25 designates a molded product. The method of shaping with use of a lathe is generally used for shaping clay into a cylindrical container with bottom. The method of shaping with a lathe will be explained.
The plaster mold 21 as shown in FIG. 5a is prepared. The clay as a raw material 22 is put into the plaster mold 21 as shown in FIG. 5b. The rotating trowel of metal 23 is pushed to the clay 22 while the plaster mold 21 is rotated, so that the clay 22 is spreaded along the inner surface of the plaster mold 21 during which water component in the clay is absorbed by the plaster mold 21, and an excessive amount of clay extending above plaster mold 21 is removed by the deburring knife 24 (FIGS. 5c and 5d). Thus, a molded product as a container is completed. Then, as shown in FIG. 5e, the molded product 25 is heated along with the plaster mold 21. Then, the absorption of water component to the plaster mold 21 and the drying of the molded product can be accelerated. As a result of the drying, the molded product 25 is shrinked as shown in FIG. 5f, whereby the molded product can be easily removed from the plaster mold 21 as shown in FIG. 5g. The removed molded product 25 is subjected to drying and calcining operations to thereby obtain the final product of container.
However, when the above-mentioned wet molding method is carried out with use of a lathe, it is impossible to form the parting grooves 2 in a surface of the molded product to which the rotating trowel is applied because it is necessary to push the rotating trowel to the inner circumferential surface or the outer circumferential surface of the molded product. Of course, the parting grooves can be formed in the surface of the molded product, which faces the corresponding surface of the plaster mold. However, when the molded product in which the parting grooves are formed only in one surface of the product, is calcined, and the calcined molded product is divided at the parting grooves, there occurs burrs in the surface where no parting grooves are formed. If the burrs come into contact with a coil portion formed on the reflection yoke, the insulating layer of the coil may be broken.
The method of producing a molded product with use of a lathe in association with a rotating trowel has the following disadvantage. Such method can be easily used for shaping a molded product of a bottomed container. However, in a case of forming a molded product of a cylindrical shape with both ends opened such as the deflection yoke core 1, the rotating trowel 23 comes to contact with the plaster mold 21 to thereby wear the plaster mold, and accordingly, stable shaping operation can not be obtained.